Banquo's ghost enriches the play in the same way the invisible floating dagger has been employed in an earlier scene. The ghost and the dagger can both be accepted as genuine supernatural events, adding mystery, danger, and suspense to the story, or they can be viewed as psychological manifestations of Macbeth's guilt, especially when considering Macbeth's reactions to the two horrible visions. The motif that guilt, deeply rooted in the mind, will manifest itself appears again in the psychological disintegration of Lady Macbeth.
It is real to Macbeth, but the ghost is invisible to everyone else at the banquet. This makes sense when you consider that only Macbeth (and the hired murderers who are not present at the dinner party) knows about his part in Banquo's murder and the attempt on Fleance's life. Banquo would only show up as a ghost in the house of the person responsible for his death.
In scene 4 of Act III to Macbeth the ghost is real, but no one else sees him. Perhaps the reason that only Macbeth sees this ghost of Banquo is the fact that it is Macbeth's conscience which is troubled. At the banquet after Banquo has been killed, Macbeth's inner thoughts give way to his fears and guilt; these emotions are manifested in the ghost of Banquo. Lady Macbeth tells him so
O proper stuff!/This is the very painting of your fear./this is the air-drawn dagger which, you said,/Led you to Duncan. O, these flaws and starts,/Impostors to true fear, would well become/A woman's story at a winter's fire,/Authorized by her grandam. Shame itself!/Why do you make such faces? when all's done,/You look but on a stool (61-69).
The supernatural motif which has run through this play several times is a popular motif for the 16th century audience of Shakespeare. The average playgoer of this time, who fervently believed in witches and the power of the supernatural, would give credibility to this ghost and the power of the witches to influence Macbeth's character and events. And, because Macbeth sees this ghost of Banquo and is concerned that Macduff has not attended the banquet, he is more inclined to seek the witches and consult them about his fate.
Ghosts are never real. They are hallucinations of terror-stricken minds. So is the ghost of Banquo appearing twice in the Banquet scene. As the First Murderer reports to Macbeth that Banquo has been killed but Fleance has escaped, Macbeth is unnerved & his fear concerning Banquo's killing and the prospect of his son to assume future kingship overpowers his apparent composure. None but Macbeth sees the ghost, but his paroxysm of fear exposes his involvement in murderous acts before the nobles. It is an irony that dead Banquo now proves a more potent threat to Macbeth than living Banquo. Banquo's ghost is an example of the psychological/subjective supernatural, very much like the air-drawn dagger that earlier led him to Duncan's bed-chamber.